The easiest and safest way is to use opto isolated triac/thyristor drivers or pulse transformer, and use a transformer to detect (and trigger an interrupt)when the mains crosses zero.
On a single phase system you can use the power supply transformer to provide the crossing reference.
The software is quite easy.
Interrupt starts/resets timer.timer waits before firing device.the longer it waits the less the power given to the load.
There are many circuit out there where the triac/thyristor are directly connected to the MCU and mains referenced .
My Best advice is to isolate the control side from the mains side.